Abstract

This paper presents a concept for conducting commercial chemical reactions and production of power using geothermal heat. The high pressures (Ps) and temperatures (Ts) that fluids attain in deep reservoirs can be used to manufacture chemicals or decontaminate wastes. High P reactions which can be expensive and/or unsafe to conduct above ground can be conducted in geothermal reservoirs using closed designs. We present examples of reactions that could benefit from Enhanced Geothermal Systems (EGS) including production of ammonia (NH₃), supercritical oxidation of wastewater contaminants, production of hydrogen (H₂) by steam reforming of methanol (CH₃OH) and partial oxidation of methane (CH₄) to produce CH₃OH.

Introduction

Geothermal energy is a huge domestic energy resource that is environmentally friendly and can provide baseload capacity to meet customer demands around the clock. It is estimated that geothermal power plants emit only between 19 and 103 g of CO₂– equivalent per kWh of electricity [10]. By comparison, coal-fired power plants emit about 1235 g of CO₂-equivalent per kWh of electricity, and natural-gas-fired power plants emit about 487 g of CO₂-equivalent per kWh of electricity [10]. A recent study by the

Massachusetts Institute of Technology [6] estimated that EGS could provide >100,000 MWe in 50 years. At present, geothermal energy is being used for power generation and for industrial and domestic heating applications. Technology to recover energy from EGS is under development. 

Presently, many challenges are facing the development of a successful EGS industry including:

• Drilling EGS wells that are deeper than 3000 m is in the experimental stage even though the oil and gas industry has been able to drill deeper wells but in lower temperature en- vironments. Exxon Neftegaz Limited has  drilled  a well  over 11 km deep [3].
• Fracturing of the rock and injecting a fluid to capture the heat are necessary results in excessive water use and loss. Hundreds of millions of gallons of water are required, which will stress competing water demands. For example, a plant that pumps 10 gal/s (38 kg/s) of water and have a 10% water loss will require over 31 million gallons of water per year to compensate for water loss only. This amount is in addition to the water consumed by the power plant.
• Water at high T and P, especially supercritical water, is a good solvent for some rock minerals. Dissolved minerals could re- precipitate as the T and P fluctuate in the EGS and in the pipes, thereby reducing the permeability of the EGS. Fluids that do not interact with the rock minerals could have advantages. 

• Seismic activity associated with hydraulic fracturing of EGS reservoirs could be a showstopper in some localities.

• deep EGS projects, where the T exceeds about 375 ◦C, can cause softening of the rocks which causes the rocks to creep and deform, which make it difficult and very costly to maintain the fractures.

Therefore, new concepts and designs may be necessary for more efficient utilization of this energy resource.

The temperatures (Ts) and high pressures (Ps) required to drive many commercial chemical reactions increases costs and decreases sustainability. Fluids injected in deep and hot Enhanced Geothermal Systems (EGS) reservoirs can reach their supercritical state without expensive and energy intensive mechanical pumping or compression. Supercritical fluids are excellent solvents and efficient media for conducting chemical reactions [4,7,8]. This makes the EGS reservoirs suitable for conducting chemical re- actions while at the same time transporting the EGS heat for power generation. Even exothermic reactions require heating to increase their reaction rates. In addition, because the reactions take place underground the cost of the system could be reduced because of less stringent criteria for high P reactors. Co-production of power and chemicals using EGS reservoirs present a unique business op- portunity. Because potential contamination of the underground or of the surface is a major concern both by the chemicals as well as contamination of the reactants and products by materials that may exist in the EGS and to prevent loss of reactants or products, closed system designs are necessary.

In this paper we selected high temperature and high pressure processes that could be considered for use with EGS reservoirs. These are discussed below.

Oxidation of contaminants in wastewater

Oxidation of hazardous contaminants in recalcitrant wastewater at high Ps and Ts is a known technology [9,11,15]. Supercritical water is known to be a good solvent for materials and a good medium for chemical reactions [11]. Wastewater can be pumped to the required P and contacted with air or O₂ to absorb enough O₂ to oxidize the contaminants and then injected in the geothermal reservoir. Solubility of O₂ in fresh water at 25 ◦C is w207, mg/L at 25 bar and increases at the rate of 8.27 mg/L for each 1 bar increase in P. Oxygen can also be added as liquid H₂O₂ at ambient T and P. As the water travels down the injection pipe its P and T increase at the rate of w96 bar and <10 ◦C per 1 km of depth. This will keep the O₂ in solution. The water is treated as its T and P increase and the treated water which carries the heat from the EGS and the heat generated by the oxidation reaction exits the production well in the supercritical state. The oxidation process converts the organic carbon in the contaminants to CO₂, the H₂ to H₂O and the halogens to acids such as HCl. After neutralizing the HCl, the supercritical water can be used to drive the turbines of the power plant. This process conserves water because the treated water can be used in the power plant and then can be used for other applications after it is analyzed for residual hazardous materials.

Production of H₂ via steam reforming of CH₃OH

Methanol is synthesized by reacting  H₂  and  CO  produced by steam reforming of CH₄. The EGS Ts are low for  steam reforming CH₄. The reforming reaction (CH₃OH(liq) þ H₂O(liq) þ Heat 4 CO₂(gas) þ 3H₂(gas)) can be used for capturing and transporting the EGS heat while producing H₂ [1,12,13]. The standard enthalpy of the reaction is about 130,000 kJ/kmol of CH₃OH e over 2.5 times the heat that water can capture when it boils. This reac- tion can occur at Ts as low as 200 ◦C using catalysts [13]. Fig. 3 shows the equilibrium composition for the reforming reaction for a CH₃OH/H₂O mole ratio of 1:2. A liquid mixture of CH₃OH/H₂O can be injected into the EGS at room T and P. The products exiting the EGS can be cooled with heat recovery to produce power. The H₂ can then be separated from the products. The unreacted water and methanol will be condensed after separating the H₂ and CO₂ and both CH₃OH and H₂O can be injected as a liquid solution after adding the required makeup amounts. The unreacted condensed CH₃OH/H₂O can be mixed with a fresh supply of reactants and returned to the EGS. The remaining stream will contain mainly CO₂. Fig. 4 shows the equilibrium conversion ratio of CH₃OH at different Ts in a 10 km deep EGS where the P will be about 670 bar. Fig. 4 shows that at Ts above about 500 ◦C over 90% conversion can be expected. Fig. 5 shows the equilibrium composition of the decomposed mixture in this EGS at different T and Fig. 6 shows the enthalpy and exergy (ideal work) gained by the steam reforming of CH₃OH. At about 600 ◦C each kmol of reformed CH₃OH results in a gain of about 150,000 kJ of exergy. At a second law efficiency of 60% this corresponds to about 25 kW of power. This output will be reduced by the exergy of the products recovered as chemicals. By comparison, one  kmol/h  of  H₂O,  when heated  to  600 ◦C  and assuming it reaches the surface at a pressure of 250 bar, would have gained 52,600 kJ of exergy. At a second law efficiency of 60% this corresponds to about 8.8 kW of power. Therefore, depending on what the reactants and products are, similar flow rates as for water are expected to be required to produce the same amount of power in addition to the chemicals that can be produced.

Production of CH₃OH via partial oxidation of methane

Methanol synthesis starts with steam reforming of CH₄ to pro- duce H₂ and CO. About 20% of the methane is used to supply heat for the process. The process consumes water and produces CO₂. Partial oxidation of CH₄ at high Ts and Ps  can  produce  CH₃OH (CH₄ þ 1/2O₂ / CH₃OH) [2,5,14]. Because the combustion of methane is thermodynamically more favorable the ratio of CH₄:O₂ must be kept high (>10). The process can also utilize natural gas where the primary reaction is with methane with assistance on initiation of the reaction from the heavier trace hydrocarbons.

However, the conversion of methane to CH₃OH is low but the selectivity toward CH₃OH can be high. Formaldehyde (CH₂O) and formic acid (HCO₂H) are the other products.

Because of the hazards associated with employing O₂ and CH₄ together this reaction is generally avoided. Careful design of this system is necessary. The following points should be addressed in the design of such a system: (a) O₂ and the CH₄ should be fed through separate pipes to prevent the combustion of the methane in the feed pipes, (b) mixing in the hot zone should be controlled such that the O₂/CH₄ mole ratio is low enough so that combustion does not occur, and (c) the products contain CH₃OH, CH₄, CH₂O, HCOOH (formic acid) and some residual O₂. Separation of these species adds to the complexity of the process design.

System design for implementing the chemical reactions

These processes can be carried out using closed double concentric pipe arrangements such as that shown in Fig. 7. The annulus of the double pipe can be fitted with an appropriate catalyst in the hot zone and/or with membranes to separate some of the products, such as H₂, in order to speed up the reactions. The inner pipe will be insulated. The reactants are injected into the annulus of the double pipe system and the products rise in the inner pipe and get processed at the surface. Pressurized water is injected through a separate pipe to fill the fractured EGS and to keep the fractures open. This water stays in the fractured rocks. The water facilitates the conduction of the heat from the rocks to the reaction zone.

When large flow rates of the reagents are required the wells may experience rapid cooling in the short term due to rapid thermal drawdown. Induced forced convention in the pressurized water will be necessary in order to enhance the heat transfer rate to the reaction zone and to maintain an adequate temperature distribu- tion in the EGS for a lengthy period of time.

The design presented in Fig. 7 illustrates how the heat is transported from the fractured rocks to the reaction zone. Water is circulated internally through the fractured rocks to capture the heat from the rocks and then delivers the heat to the chemically reacting materials. The difference between this design and the conventional design, as far as heat transfer is concerned, is that in the design shown in Fig. 7 the heat is recovered from the water injected into the fractures by the reaction in the EGS (underground) instead of bringing the hot water to the surface and then recover the heat from the reaction products above ground. Therefore, the heat transfer characteristics from the EGS to the chemically reacting working fluid should not differ much from the conventional case. The chemically reacting fluid captures the heat from the water that is injected in the EGS as chemical energy, sensible heat and latent heat. Therefore, more heat is transferred per unit mass or mole to the surface that what water can transfer. The flow rate will be determined by the amount of heat required to produce the required electrical power and/or the necessary chemical production rates.

The double pipe design will ensure that the chemicals will always be contained and will not be allowed to leak out of the exterior pipe. Corrosive materials in a high temperature and pres- sure environments dictate that corrosion resistant casing materials be used in order to prolong the service life of the pipes. In this application the casing material has to also have good heat transfer materials. Research is underway to develop such materials at reasonable cost. Such materials will also be required in deep high temperature EGS reservoirs regardless of what the working fluid is. As it is well known separating one of the products will result in speeding the reaction rate. Products such as H₂ can be separated in- situ at the bottom of the inner pipe of the double pipe by incor- porating an appropriate membrane sleeve (such as palladium membranes for H₂ separation). Catalysts can also be incorporated in the design (Fig. 8). Both of these will require more frequent maintenance. This may require bringing the inner pipe (in sections) to the surface to maintain the membrane and the catalyst when required.

Economic analysis

Detailed economic analysis to establish economic feasibility will be necessary before this concept can be implemented. However, reliable economic analysis can be conducted when the design and operation specifics of the EGS and the chemical and power pro- cesses are known. At this stage this paper presents a concept. A brief discussion of its potential economic benefits is given below.

In the conventional case two wells will have to be drilled: an “injection” well and a “production” well. Water is injected into the EGS through the injection well and is circulated through the frac- tured EGS in order to get heated. The heated water reaches the “production” well and rises in it to the surface.

In the proposed concept (Fig. 7) two wells are also needed. However, one of the two wells is a smaller diameter well for injecting water into and circulating it through the fractured zone. The water will be kept underground and therefore only a small diameter pipe will be necessary for injecting this water. The second will be fitted with a double pipe. The outer pipe will be closed at the bottom and will be used for injecting the reactants through the annulus of the double pipe. The internal pipe is open at the bottom and will be used for bringing the products of the reaction to the surface. The reacting fluids will capture the heat from the circu- lating water underground. Because the reacting fluids will capture the heat as chemical energy in addition to sensible and latent heats the flow rate of the reacting fluids for the same amount of heat capture can be smaller than that for water. Therefore, the diameter of this well may also be smaller than the conventional well and may cost less. The cost of the internal pipe in the double pipe will be relatively very small. Therefore, overall the drilling and completion costs can be lower than the costs for a conventional system.

Heat is recovered from the hot products at the surface and is used for power generation and the chemical products are recovered for use as chemicals. The products which will reach the surface as hot and at high pressure gases can also be used directly as the working fluids in the power cycle. For example, when NH₃ is the product of the reaction of N₂ and H₂, the products can be expanded to produce power and the low pressure ammonia that exits the expander can be recovered as a product. Therefore, the cost of the wells will be split between the power plant and the chemical process. Because some reacting chemicals can capture significantly more heat per unit mole or volume than water, the system can be designed to produce the required power and chemicals with the same flow rate or even less.

In addition, the reactions will be carried out underground without having to pump or compress the reactants to very high pressures and no expensive high pressure reactors or heat ex- changers will be required. Another benefit is that high pressure safety issues associated with above ground operation which will require special containment structures will not be required and this will reduce the cost also.

Conclusions

Combined power generation and production of useful chemicals using EGS reservoirs present an opportunity for more efficient utilization of geothermal energy. These reactions capture EGS heat as chemical energy as well as sensible and latent heat and there- fore, can capture significantly more heat per unit mass of flow than water. Costs, safety and environmental impacts present distinct challenges for the deployment of these fluids. Other renewable heat sources, such as concentrated solar, could also provide the energy for driving chemical reactions. Developing a more detailed model of the overall process performance and economics will assist the industry in realizing this opportunity.